1. Field of the Invention
This invention relates to a power steering device for vehicles, and more particularly to a variable-steering-ratio power steering device in which the steering ratio, the ratio of the rotating angle of the steering wheel to the turning angle of the dirigible wheels is variable relative to the speed of the vehicle.
2. Description of the Prior Art
There has been known a power steering device which is provided with a power assisting device (hereinbelow referred to simply as "power assist") in the steering linkage thereof to reduce the effort required to rotate the steering wheel. Generally, the steering ratio is preferred to be relatively small while the vehicle is at rest or moving at a low speed so that the dirigible wheels can be turned by a relatively large angle with a small rotating angle of the steering wheel, thereby reducing the steering effort of the operator. On the other hand, the steering ratio is preferred to be relatively large while the vehicle runs at a high speed in order to prevent oversteering and to minimize the influence of sidewinds, road shocks and the like, thereby providing steering stability. However, in the conventional power steering devices in which the steering ratio has a fixed value over almost the entire speed range, it is impossible to meet both the requirements. Therefore, the steering ratio is conventionally fixed at an intermediate value as a compromise between the two requirements.
Recently, there has been proposed in U.S. Pat. No. 4,310,063 a power steering device in which the steering ratio is made variable according to the speed of the vehicle. In the power steering device, resilient means in the form a torsion bar is inserted into a portion of the steering linkage between the steering wheel and the power assist which portion transmits the steering force from the steering wheel to the power assist as a rotary motion thereof. When the steering wheel is rotated, the torsion bar is elastically deformed to produce a rotational displacement between portions connected to opposite ends thereof, the amount of the elastic deformation of the torsion bar depending upon the steering force of the steering wheel whereby the steering ratio is changed according to the speed of the vehicle. That is, when the vehicle is running at a low speed, the torsion bar is elastically deformed by a relatively small amount whereby the steering ratio is kept relatively small, while when the vehicle is running at a high speed, the torsion bar is elastically deformed by a larger amount whereby the steering ratio is made larger.
However, the power steering device as disclosed in the above U.S. Patent is disadvantageous in that the free play range or the insensitive range at the central portion of the steering range is excessively widened since the free play due to the elasticity of the resilient means inserted into the linkage is added to the mechanical free play in the linkage itself, e.g., the free play in the joints of the linkage and the free play in mesh of a rack and a pinion. Excessive free play in the steering system adversely affects the response in compensating for external disturbances and in recovery of the front wheels when running path is changed.
A further disadvantage of the power steering device of the above U.S. Patent is that the variable range of the steering ratio is relatively narrow since the torsion bar employed as the resilient means has a small allowable twisting angle (amount of the rotational displacement) and the degree of the freedom in selecting the characteristics of the torsion bar is limited. Further, the divice is complicated in structure and is large in size.
Further, in the power steering device of the above U.S. Patent, when an excessive load is exerted on the resilient means as is the case when the steering wheel is operated while the vehicle is at rest or the steering wheel is sharply rotated to avoid an obstruction in the road, the resilient means may be broken. In order to avoid the possibility of breakage of the resilient means, a restraining means is provided to limit the amount of rotational displacement of the steering linkage to a predetermined angle, thereby limiting the load exerted onto the resilient means. However, the restraining means involves a problem in that the steering effort required to operate the steering wheel abruptly becomes heavier to give the operator a discontinuous feeling in steering when the restraining means comes into action.
Generally, the steering ratio is preferred to be large when the vehicle is running at a high speed in order to prevent oversteering. However, expert drivers or those who are fond of sharp steering response may want the steering ratio to be small even when the vehicle is running at a high speed. Further, different steering ratio characteristics will be appreciated when driving in town and in the country. Thus, it will be convenient if the steering ratio characteristics can be adjusted freely according to driver's choice.